Many existing biological and chemical sensors detect the presence or absence of target material in the vicinity of the sensor based on changes in the propagation of electro-magnetic radiation. Such sensors have previously been fabricated using dielectric waveguide components on the basis of silicon-on-insulator (SOI) wafers and using metallic surfaces that support surface plasmon polaritons, i.e. electromagnetic excitations at a metal-dielectric interface. While such sensors will typically meet sensitively requirements for useful sensors, their dimensions are limited by diffraction effects in the dielectric waveguide components used and by the coherence length of surface plasmon polaritons, which limits the area on a metal surface that can considered to represent an independent measurement. Such sensors can be therefore less attractive for integration with silicon based electronic components, which are many times smaller.
For example WO2009/112288 describes plasmonic sensing devices that are based on “extraordinary optical transmission” (EOT) phenomena in which the normalized transmission cross-section of sub-wavelength holes or slits arranged in an array exceeds that of a single sub-wavelength hole or slit. These effects have been extensively studied since the first report of Ebbesen, et al., in Nature, February 1998. The EOT effect can be attributed to resonances between light that is directly transmitted through the holes and light that is scattered into and out of surface waves by the sub-wavelength features, in some cases including surface plasmon polaritons that are confined to the exterior metal-dielectric interfaces of the structure supporting EOT.
One problem with EOT plasmonic sensing devices is that they will be subject to constraints deriving from the coherence length of single interface propagating surface plasmons, which is typically of the order of a few micrometers. A second problem with such devices is that they will require at least two scattering centers to establish the interference condition necessary for EOT. Hence in order to accomplish EOT effects typically an EOT-supporting metallic grating (e.g. an array of plasmonic scattering centers provided in a metallic layer or multilayer) of relatively large dimensions is needed so that a high integration density cannot be accomplished.
A further plasmonic sensing device is described in U.S. Pat. No. 7,599,066. This document describes a sensor that relies on detecting absorption features associated with the localized plasmon resonance of nano-sized metallic particles. Such particles however are irregularly shaped and thus not very well defined resulting in deterioration of the resonant response of the sensor. Moreover, the irregularly will also affect the uniformity of the sensors when fabricated as dense sensor arrays on a wafer.
More recently, alternative waveguide structures that support surface plasmon polariton-like modes have been developed consisting of metal-dielectric-metal waveguide structures, such as those described by Dionne, et al., Physical Review B 73, 035407, 2006. These excitations propagate with an effective wavelength that is smaller than the effective wavelength of surface plasmon polaritons that are supported by a single metal-dielectric interface of the same frequency. Such metal-dielectric-metal waveguide structures further allow for electrically driven discrete devices for generating surface plasmons. Such devices are known for example from the article by Walters et. al., “A silicon-based electrical source of surface plasmon polaritons”, Nature Materials, 6 Dec. 2009.
Hence, improvements are needed in order to achieve integrated plasmonic sensing devices based on localized plasmon resonance mechanisms. In particular, improvements are needed for the realization of integrated plasmonic sensing devices based on well-defined localized plasmon resonances, wherein the formation of the sensing devices is compatible with CMOS fabrication and wherein the formation allows dense integration of sensitive biological and chemical sensors and other optical spectroscopy tools on a silicon wafer.